Kidney and heart transplants will be performed between highly defined strains of inbred mice. Further explorations will be conducted of the mechanism whereby such transplants in certain H-2 incompatible mouse strains will servive for many months supporting life, in the case of kidney transplants which are placed in nephrectomized recipients. Long-term transplant recipients have been shown to be deficient in "killer cells" directed toward donor specific antigens but to contain lymphoid cells fully capable of mounting an immune response to these antigens on transfer to other animals or in cell culture. Efforts will also be made to increase the immune response of such recipients to donor antigens by various programs of immunopotentiation.